Biological sludge disposal is a growing worldwide problem. In the United States, sewage sludge production is in excess of 20,000 tons/day and disposal costs have increased sharply due to bans on ocean dumping and decreasing landfill capacity.
The unlimited resource of sewage sludge can be processed for use as a soil conditioner or fertilizer. In the United States, Environmental Protection Agency (EPA) regulations must be met with regard to remaining pathogen levels before processed sludge can be used in this manner and only a small fraction of the generated sludge is converted to this end product.
Composting has also been used as a disposal method for sewage sludge. Methods for producing a compostable mixture of sewage sludge and composting are disclosed in U.S. Pat. No. 4,659,472. A material such as sawdust is mixed with wet sewage sludge in the presence of air. The mixture is supplied with a polyelectrolyte solution and then pressed to a solids content of about 30 percent by weight. The resultant compostable mixture can be subjected to a composting process in containers having a 15-20 m.sup.3 volume. The containers have a device for supplying and distributing air through the mass of compostable material present in the container. Temperatures of 50.degree. C. (122.degree. F.) are obtained after 24 hours and temperatures of 50.degree. to 60.degree. C. (122.degree. to 140.degree. F.) for a period of seven days are used to destroy pathogenic microorganisms.
U.S. Pat. No. 4,392,881 discloses a process for composting sewage sludge using two processing steps. Waste material is introduced into a first closed vessel and continuously aerated from the bottom. The material in the vessel is discharged from the bottom such that it is resident within the vessel for 7 to 14 days. Material discharged from the first vessel, now biologically active, is transferred to a second closed vessel and aerated discontinuously. The material is resident in the second vessel for 14 to 20 days. The final product is disclosed to be useful as fodder.
Sludge has also been disposed of by combustion. A rapid expansion of this technology began in the late 1950s and continued through the late 1970s.
Processing of sewage sludge for use as a fuel has been disclosed in U.S. Pat. No. 1,064,773 which discloses a process of dewatering and using raw sewage by mixing it with finely divided active carbon and then aerating the mixture with an oxygen containing gas. The aerated material is then dewatered. The material can be pressed into cakes and used as a fuel.
U.S. Pat. No. 4,552,666 discloses adding dried autumn foliage or extracts thereof to digested sewage sludge to sediment the sludge into deformable materials which can be shaped into briquettes for use as a fuel.
U.S. Pat. No. 4,828,577 discloses a method for burning food preparation wastewater sludges. The fuel value can be increased by adding a bulking agent such as sawdust.
At its height, approximately 5,060 tons/day out of 18,750 tons/day solids produced by U.S. wastewater treatment plants were being combusted. However, the technology has fallen into disuse due to high costs associated with sludge processing to achieve a moisture content which results in a heating value competitive with other waste fuels such as anthracite culm. Accordingly, a need exist for a facile, efficient and inexpensive method to produce a low sulfur or sulfur free combustible fuel product from biological sludge with a heating value competitive with currently used waste fuels.